Known power storage devices include secondary batteries, such as lithium ion batteries, nickel hydride batteries, and lead batteries, and electrochemical capacitors, such as electric double layer capacitors. Due to miniaturization of mobile devices, limitation of installation spaces, or the like, further miniaturization of power storage devices is sought, and thus attention is given to lithium ion batteries for their high energy density.
Metal cans that have been used for packaging materials for lithium ion batteries are being replaced by multilayer films (e.g., configuration including substrate layer/metal foil layer/sealant layer) due to their light weight, high heat dissipation, and low manufacturing cost.
Such a lithium ion battery using a multilayer film as a packaging material uses a configuration in which battery contents are covered with a packaging material including an aluminum foil layer, serving as a metal foil layer, to prevent moisture from penetrating into the battery. Lithium ion batteries using such a configuration are referred to as aluminum laminated lithium ion batteries. In addition to a positive electrode, a negative electrode and a separator, lithium batteries contain an electrolyte or an electrolyte layer formed of a polymer gel impregnated with the electrolyte. This electrolyte is prepared by dissolving a lithium salt in an aprotic solvent having a penetrative ability, such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate.
Embossed lithium ion batteries, for example, are known as aluminum laminated lithium ion batteries. Such an embossed lithium ion battery is prepared by forming a recess at part of a packaging material by cold forming, storing battery contents in the recess, folding back other part of the packaging material, and heat-sealing the edge portions. The packaging material configuring such a lithium ion battery is required to exhibit stable sealing performance after being heat-sealed and to be less liable to lose the lamination strength between the aluminum foil layer and the sealant layer due to the electrolyte of the battery contents.
With miniaturization of power storage devices, thickness reduction of the substrate layer, the metal foil layer and the sealant layer of the power storage device packaging materials is underway. However, thickness reduction of the sealant layer poses a problem of deteriorating insulation properties.
Deterioration in insulation properties is induced by various phenomena. Examples of the phenomena include contact between the tab lead and the metal foil layer due to the heat generated during heat sealing or the like, development of cracks in the sealant layer due to forming, bending or the like, fusion of portions which should not be fused during heat sealing (hereinafter called excessive sealing), and damage of the sealant layer due to degassing heat sealing.
In this regard, for example, PTL 1 suggests a packaging material having improved insulation properties and exhibiting more stable sealing performance, heat resistance and formability. This packaging material is provided with a heat-seal layer (sealant layer) that includes a high-melting-point polypropylene layer with a melting point of 150° C. or more and a propylene-ethylene random copolymer layer.